Rapidly solidified metal powers are made up of fine crystal grains and can be supersaturated with alloy elements, so that the extruded material prepared, for example, from a rapidly solidified powder of aluminum or an alloy thereof is superior in characteristics to the material prepared from a molten metal and has attracted attention as a material for machine parts and the like.
The preferred methods of producing such rapidly solidified metal powders include the rotary drum method. With reference to FIG. 10 showing this method, a stream of molten metal is injected into a cooling liquid layer 62 centrifugally formed over the inner peripheral surface of a rotating cooling drum 61 to finely divide the molten metal and obtain a rapidly solidified metal powder. Indicated at 63 in the drawing is an injection crucible serving as means for injecting the molten metal and provided with a heating high-frequency coil 64 around its outer periphery and an injection nozzle 65 in the lower portion of its side wall. The crucible 63 contains the molten metal 66, which is forced out from the nozzle 65 by injecting an inert gas 67 into the crucible 63 under an increased pressure. When a predetermined amount of metal power accumulates in the cooling drum 61, the rotation of the drum 61 is stopped, and the powder is collected along with the cooling liquid, followed by removal of the liquid and drying. Examined Japanese Patent Publication HEI 1-49769 discloses such a method of producing metal powders.
However, the rotary drum method is practiced by a so-called batchwise operation and is low in productivity. Additionally, the need to discontinue the injection of molten metal for collecting the powder entails the problem that the nozzle orifice is prone to clogging.
Further to maintain a constant cooling temperature, the cooling liquid must be supplied to and discharged from the liquid surface of the cooling liquid layer for temperature control, whereas this disturbs the liquid surface and gives rise to the problem that variations are liable to occur in the particle size or quality of the powder.
Since the powder is collected along with the cooling liquid, the method has another problem in that the removal of the liquid requires a considerable period of time thus resulting in a poor efficiency. Moreover, the powder is held in contact with the cooling liquid for a prolonged period of time and therefore contains an increased amount of hydrogen, oxygen or like gas, which is likely to produce defects in the material to be obtained by extruding the powder or by heat-treating the extrudate.